1. Field of the Invention
The present invention relates to a printing apparatus and a printing position adjusting method thereof.
2. Description of the Related Art
Printing apparatuses that adopt an ink-jet printing method are known. Such printing apparatus prints an image on a printing medium by discharging inks from orifices arrayed on a printhead while reciprocally moving the printhead.
In recent years, in such printing apparatus, the number of orifices (nozzles) tends to increase so as to increase a printing speed. The printing apparatus includes a printhead having a plurality of orifice arrays (nozzle arrays) in correspondence with a plurality of ink colors, so as to implement color printing.
Under such circumstances, dot printing positions between nozzle arrays are often shifted due to, for example, shifts of nozzle forming position and that of printhead mounting position at the time of manufacture of the printhead. When a plurality of printheads are used, dot printing positions are often shifted due to a relative position shift between the printheads. Furthermore, a single nozzle array often causes shifts of dot printing positions when printing is done in two directions (forward and backward directions).
To solve these problems, printing position adjustment processing for adjusting dot printing positions (also called registration processing) is known. In the registration processing, by selecting one nozzle array as a reference, a relative shift amount of the dot printing positions by another nozzle array with respect to those by that nozzle array is calculated, and ink discharge timings are corrected based on the shift amount. As for shifts of the dot printing positions in forward and backward print scans at the time of bidirectional printing, the registration processing is similarly attained by correcting discharge timings.
In this case, when an adjustment value required to adjust dot printing positions is to be calculated, for example, a plurality of shifted patterns (obtained by shifting printing positions little by little from those of reference patterns) are printed using another nozzle array to overlap the reference patterns printed using a reference nozzle array. Then, a shift amount of ink landing positions of dots is detected based on density changes with respect to a shifting amount of the patterns which are printed to overlap the reference patterns, and the ink landing positions of dots are corrected using this shift amount.
As a method of detecting the shift amount, for example, visual confirmation by the user is known. Note that when the aforementioned adjustment value is calculated at the time of bidirectional printing, the user visually confirms print results obtained by printing a plurality of patterns while shifting discharge timings in a backward scan with respect to a forward scan. The visual confirmation forces troublesome works on the user. Hence, a technique which optically reads adjustment patterns using a sensor, and controls the apparatus to automatically calculate an adjustment value based on the reading result has also been proposed (Japanese Patent Laid-Open No. 10-329381).
In recent years, since a droplet size reduction of an ink to be discharged has progressed for the purpose of improvement of printing quality, disturbances influence heavily on ink discharging and dot printing. The disturbances include, for example, vibrations upon movement of a carriage which mounts a printhead, and posture variations of the printhead upon scanning the carriage due to a distortion of a rail stay which supports the carriage.
Such disturbances cause variations of dot printing positions at the time of printing of adjustment patterns, and may influence formation of patterns used in the registration processing. In order to suppress the influences of these disturbances, a measure such as enhancement of mechanical precision of a printing apparatus may be taken, but it is not desirable in terms of cost.
It is demanded to correctly execute adjustment even when such variations of dot printing positions have occurred. A technique described in Japanese Patent Laid-Open No. 2006-102997 refers to a technique which prints abnormality detection patterns in synchronism with adjustment patterns, and corrects read values of the adjustment patterns influenced by the disturbances at the time of adjustment or executes calculations by excluding the influenced patterns at the time of calculations of the adjustment value. Also, a technique described in Japanese Patent Laid-Open No. 2009-39916 refers to a technique which calculates an adjustment value by interpolation based on tendencies of a plurality of patterns, and changes the number of patterns used in the interpolation by checking whether or not a pattern having a different tendency is included upon execution of the interpolation, thereby reducing the influences of the disturbances.
However, with the techniques described in Japanese Patent Laid-Open Nos. 2006-102997 and 2009-39916, a pattern itself cannot be normally formed due to the disturbances, and a plurality of patterns cannot have intended density changes. For this reason, a problem about an unavoidable certain precision drop remains unsolved.